In Pennsylvania, the controversial practice of fracking can consume 4.5 million gallons of water per well, the liquid pumped deep underground to crack rocks that contain natural gas.

In parts of Texas, fracking a well often takes 6 million gallons.

But in California, where fracking is starting to spread, the average amount of water involved is just 164,000 gallons, according to industry data.

Hydraulic fracturing, known as fracking, has triggered a boom in energy production across the United States and sparked a fierce public debate that revolves around water. Critics say fracking can ruin drinking water supplies when badly built wells allow chemicals used in the process to seep into aquifers. The disposal wells that take used fracking water and bury it far beneath the earth's surface can trigger earthquakes.

And in arid Western states, the sheer volume of water that fracking requires alarms farmers and environmentalists alike.

"Here in California, as much as people worry about contamination, water supply and induced seismicity are at the front of people's minds," said Damon Nagami, a senior attorney with the Natural Resources Defense Council environmental group. "The lack of data on these issues is freaking people out."

Less water used in California

But so far, fracking in California appears to take far less water than it does elsewhere.

At the request of state regulators, some of the companies fracking here have started posting information about their wells on FracFocus.org, a website created by the oil and gas industry to allay public fears about the practice. The site contains information on 364 fracked wells in the Golden State, most of them in the southern San Joaquin Valley. (For comparison, FracFocus lists 1,940 fracked wells in Pennsylvania - ground zero of the fracking boom.)

The average amount of water used in California wells has been 164,000 gallons, according to the Western States Petroleum Association, an oil industry lobbying group that compiled a spreadsheet of the California data. Some fracked wells here require significantly more - 300,000 gallons and up - while others consume substantially less. A cluster of fracked wells near Sutter Buttes in the Sacramento Valley used between 10,000 gallons and 35,000 gallons apiece.

An Olympic-size swimming pool contains about 660,000 gallons of water. A golf course typically uses around 300,000 gallons per day.

"We're talking about pretty small-scale operations relative to what we've seen in the rest of the country," said Tupper Hull, the petroleum association's spokesman.

Why the difference? Geology.

In much of the United States, fracking has been combined with horizontal drilling to unlock natural gas or oil within shale rock. Imagine drilling straight down for 5,000 or 6,000 feet, then taking a right turn. Water is pumped into the well at high pressure, mixed with sand and a small amount of chemicals (usually about 1 percent of the mix). The pressure cracks the shale, and the sand props those cracks open, allowing oil or gas to escape.

In California, most fracking has involved vertical wells. Vertical wells have less pipe length than horizontal wells of equal depth, because they don't veer off sideways at the bottom. They therefore require less water for fracking.

So far, horizontal wells don't appear to work well in the state's Monterey Shale formation, estimated by the federal government to be the country's largest oil shale deposit. The rock layers are too tilted and folded by relentless seismic pressure to lend themselves to horizontal wells.

"California has been folded and faulted quite a bit," said Tim Kustic, head of the Division of Oil, Gas, and Geothermal Resources, which oversees drilling. "Depending on where you are, the same formation can be deep in one spot and outcropping on the surface 100 miles away. If it's tilting, horizontal may not be the best way to go."

Oil, not gas, the goal

In addition, most fracking in California is for oil, not natural gas. And the oil typically resides in rock formations that contain large amounts of brackish water, Kustic said. Since the pores within the rock are already full of liquid, drillers don't need to add much to increase the pressure.

"You contrast that with shale gas - the reservoir is quite dry," he said.

The amount of water pumped into a fracked well does not appear to have a direct link to the risk of contaminating drinking water supplies. Most of the cases of tainted water blamed on fracking have involved chemicals and methane migrating along the exterior of poorly constructed wells - a problem that can happen whether the well has been fracked or not.

Although the use of fracking appears to be growing in California, it hasn't taken off the way it did in Pennsylvania, Texas and North Dakota. The Monterey Shale is estimated to hold up to 15.4 billion barrels of recoverable oil, giving companies a powerful incentive to keep drilling. The country uses about 19 million barrels per day.

The amount of water needed for fracking could grow if the practice becomes widespread, or if horizontal drilling proves more useful here than it has to date. At this point, however, the amount of water involved appears relatively small.